Pumps according to the above are used for example for producing lubrication oil pressure in combustion engines for vehicles. The oil is transported by pressure to various parts of the engine and is used for lubricating a large number of bearings and hydraulic engine components.
It is normally required of an oil pump that it should give the required pressure even at the idling speed of the engine. Thus at higher speeds an unnecessarily high oil pressure is produced, which has to be reduced in some way, e.g. via an overflow valve.
The pump is normally driven by a power take-off from the engine drive shaft and the overproduction of oil pressure therefore has an effect on both the fuel consumption and the usable power. A variable displacement pump should be able to reduce the power requirement especially at speeds above idling. Such pumps are known but present disadvantages since they are comparatively expensive and complicated.